Multiple Input—Multiple Output (MIMO) transmission modes are a common feature of state-of-the-art wireless communication systems. They are in particular basic to the mobile communication standards Evolved Universal Terrestrial Radio Access (E-UTRA) (Long Term Evolution (LTE)), Worldwide Interoperability for Microwave Access (WiMAX) and High-Speed Packet Access (HSPA) but also relevant for Wireless Local Area Network (WLAN) communications and in other applications. Explanations of MIMO transmission modes can be found in “Introduction to space-time wireless communications” by Paulraj et. al, ISBN 0 521 82615 2.
So as to support MIMO transmission modes such as Receive Diversity (RD) and Spatial Multiplexing (SM), a mobile terminal or wireless modem must be equipped with at least two antennas, providing a two port antenna system, and in the general case a multi port antenna which is designed to receive (or transmit, respectively) signals from different ports independently of each other in the same frequency band. For receive mode “independently” means that the different ports of the multiport antennas are capable of receiving different superpositions of incoming multi-path components which requires that their polarimetric, complex radiation patterns are sufficiently distinct. A quantitative measure of this capability is the correlation between signals received at different antenna ports in a given reference propagation scenario.
An often applied metric is the Complex Envelope Correlation Coefficient (CECC) between two antenna signals in the isotropic Rayleigh propagation scenario. Due to the principle of reciprocity which applies to antennas the statements apply analogously to transmit mode operation.
A key performance metric of an antenna for a mobile terminal or wireless modem is its bandwidth at a given desired frequency of operation. Other important metrics like radiation efficiency are for physical reasons strongly correlated with bandwidth (provided that good engineering practices are applied). The bandwidth of an antenna is physically limited by its size. If the physical size of an antenna becomes much smaller than the free-space wavelength of an electromagnetic wave at the frequency of operation, the bandwidth of an antenna decreases roughly in proportion to the third power of its largest dimension. For a multiport antenna for MIMO transmission, the smallest bandwidth seen at any of its ports, will limit the MIMO operation. In the context of mobile terminals and wireless modems the largest dimension is to be understood as the largest dimension of the combined arrangement of the nominal antenna and the conductive structure of the device (e.g. mobile phone or laptop) to which it is attached and on which a current density may be excited by the antenna (transmit mode) or to whose excitation by an incoming electromagnetic wave it couples (receive mode). This coupling between antenna and conductive chassis is essential for almost all practical small mobile devices. Their antennas would not provide the required bandwidth otherwise.
The exploitation of the conductive chassis of a mobile device as an antenna extension can be described as the excitation of characteristic modes of the chassis. The radiation pattern of the antenna-chassis combination is given by the respective characteristic modes. So as to achieve the above mentioned “independency” coupling) between multiple antenna ports in a MIMO antenna system for a mobile terminal or wireless modem, different superpositions of characteristics modes must be excited (transmit mode) by different antenna ports. This is relatively easy to achieve if the structure is “large” (at the order of half a wavelength) in at least two dimensions and if antenna elements can be spaced at sufficiently “large” distances along the periphery of the conductive chassis.
Unfortunately this is not the case for small size wireless modems such as e.g. realized in Express Card format or in the form of a Universal Serial Bus (USB) dongle attached to a laptop particularly at frequencies below 1 GHz. The antenna within the modem is then located within a single small volume at one edge of the laptop. The result is that one antenna port shows a wide bandwidth while a second or further ports show relatively small bandwidth. In particular it turns out that the required instantaneous bandwidth for MIMO coverage of low-frequency E-UTRA band classes is not attainable within the physical size of an Express Card or USB stick.
For the realization of efficient multi port antenna systems in such devices it is therefore mandatory to extend the volume for a second or further antenna ports over the limits of an Express Card or USB stick. Unfortunately, this approach leads to bulky devices and is in conflict with customer expectations and typical use cases for mobile devices such as use of a laptop in a train.
The problem was less severe in the past when multiport antenna systems in mobile devices were predominantly designed for frequencies of operation near 2 GHz. It is a more severe problem now as frequency bands in the range from 698 MHz to 960 MHz are employed for MIMO transmission. In existing devices (mainly designed for RD and not for SM mode and mostly at higher frequencies) the concept of a primary (good) and a secondary (poor) antenna is often followed. But for SM operation all antennas should have comparable performance in terms of bandwidth and efficiency.
In some other “MIMO” devices, only a single antenna is integrated within the wireless modem and an external connector is foreseen to attach a second antenna. This solution must be rated as extremely inconvenient from a user's point of view since he/she has to carry and assemble additional equipment for mobile use. It is makes matching more difficult.